Thermocouple assembly with simple, easy to change, good seismic performance, high measuring accuracy, large measuring range, fast response time, hot pressure good performance, long life and other characteristics, is commonly used in temperature measuring instrument of temperature measuring element, it directly measure temperature, and the temperature signals into a thermoelectric emfs, through electrical instrument ( Secondary instrument) Convert the temperature of the measured medium. At present there are several commonly used thermocouple 1, platinum rhodium/platinum thermocouple - — The index number is s, the anode is an alloy of 10% 90% platinum and rhodium, negative extremely pure platinum wire. The advantage of the thermocouple is easy to prepare high purity platinum rhodium, so easy to copy, and the temperature measurement of high precision, can be used as international practical scale in 630. 74 - 1064. 43 ℃ within the scope of the benchmark thermocouple. The physical and chemical stability high, appropriate use in oxidizing and neutral atmosphere; Its high melting point, high temperature limit is. In the industrial measurement generally use it to measure the temperature of above 1000 ℃, under 1300 ℃ can be used for a long time continuously, short-term WenKeDa 1600 ℃. Platinum rhodium/platinum thermocouple disadvantage is that the price is expensive, thermoelectric power is small, the reducing gas, metal, metal oxide, and silicon oxide and sulfur oxide atmosphere use will soon be contamination and deterioration, so he must use it in the atmosphere and protect casing, in addition, the thermocouple thermoelectric performance of nonlinear is larger, the hot electrode can sublimate at high temperatures, make rhodium molecules penetrate into the platinum extremely contamination, it lead to thermoelectric potential instability. 2, nickel chromium/nickel silicon thermocouple - — Its index number is k, the positive component is 9 - Chromium, 0 10%. 4% of silicon, the rest of nickel, anode component of 2. 5 - 3% silicon, 3, nickel chromium/constantan thermocouple - — Index number is e, the anode nickel chrome components in 9 - 10% chromium, 0. 4% silicon, the rest is nickel; The cathode constantan 56% copper and 44% nickel. Big advantage of nickel and chromium/constantan thermocouple is thermoelectric potential is big, the price is cheap. The shortcoming of the thermocouple is cannot be used to measure the high temperature, the temperature up to 800 ℃, use for a long time, will only be below 600 ℃, in addition, due to test copper alloys are susceptible to oxidation and deterioration, must be equipped with protective casing. 4, platinum rhodium 30 / platinum-rhodium thermocouple 6 - — Referred to as double platinum rhodium thermocouple, index number is b. Across the electrodes of the thermocouple is platinum rhodium, just different proportion alloy content and the third, the anode containing rhodium 30%, anode containing rhodium is 6%, double platinum rhodium thermocouple of fouling resistance is strong, when the temperature of 1800 ℃ temperature is still a good stability. The temperature measurement accuracy is higher, suitable for oxidizing, neutral medium, can long-term continuous measuring 1400 - The high temperature of 1600 ℃, short-term measure up to 1800 ℃. Double platinum rhodium thermocouple sensitivity is low, when using, should with high sensitivity digital display table. At room temperature for thermoelectric potential minimal impact, so when using do not generally requires temperature compensation. 5, copper constantan thermocouple / - Its index number for t, is extremely copper, negative extremely 60% / 40% copper nickel alloy. Its advantage is high temperature sensitivity, thermal electrode is easy to copy, cheap, good low temperature performance, can be measured, 200 ℃ low temperature. But its composition is easy oxidation of copper, so the average maximum temperature does not exceed 300 ℃.